The present invention relates to a heat-sensitive transfer recording medium.
In general, a heat-sensitive transfer recording medium is formed by applying an ink composed of a vehicle composed of a wax, a resin, etc. and a colorant onto a substrate. Depending on states at the time of application, the ink for use in the heat-sensitive transfer recording medium is mainly classified into a hot-melt ink containing no solvent and a solvent ink wherein a vehicle and a colorant are dissolved or dispersed into a solvent. The solvent ink has a greater degree of freedom in material selection, which makes it possible to provide inks having various characteristics. However, since a solvent is used, the material costs become higher correspondingly, and it also requires drying costs, resulting in higher costs, as compared with the hot melt ink.
In contrast, the hot-melt ink, which uses no solvent, is more advantageous as compared with the solvent ink. However, since the application of the hot-melt ink is conducted by a hot-melt coating method, the upper limit of the melt viscosity of the ink is restricted with the result that usable materials are limited, and it is difficult to obtain inks having various characteristics. For example, in the case of black ink, due to the restriction in the melt viscosity, it is necessary to use carbon black having a smaller DBP oil absorption as a colorant. However, the hot-melt ink without containing any solvent which uses carbon black having a small DBP oil absorption tends to cause poor conductivity in the ink layer after application, and troubles due to static electricity.
Moreover, even when an attempt is made to use conductive carbon black so as to solve the problem with the hot-melt ink containing no solvent, the conductive carbon black has a great DBP oil absorption and a great specific surface area, with the result that the melt viscosity of the hot-melt ink becomes greater, causing degradation in the coating property; thus, it has been considered that it is impossible to put this into practical use.
On the other hand, a method has been proposed in which, in a heat-sensitive recording medium using a hot-melt ink containing no solvent, a mat layer is placed between the substrate and the hot-melt ink layer so as to improve the readability of a printed image so that the gloss of the printed image is reduced and a conductive powder is used as a mat agent for the mat layer so as to prevent the troubles due to static electricity. However, it is necessary to add a considerable amount of the conductive powder to the mat layer in order to achieve sufficient conductivity in a thin film such as the mat layer. This causes a reduction in the strength of the mat layer, resulting in separation of the mat layer at the time of printing depending on printing conditions, and failure to obtain gloss-reducing effects in the printed image.
In view of the above-mentioned problems, an object of the present invention is to provide a heat-sensitive transfer recording medium which can prevent the occurrence of troubles due to static electricity without impairing the coating property of the hot-melt ink.
This and other objects of the present invention will become apparent from the description hereinafter.
The present invention provides a heat-sensitive transfer recording medium comprising a substrate and a heat-sensitive transfer ink layer provided on the substrate, the heat-sensitive transfer ink layer being a coating of a hot-melt ink containing no solvent therein, the heat-sensitive transfer ink layer containing carbon black having a DBP oil absorption of 100 to 200 ml/100 g and a BET specific surface area of 100 m2/g or less, and the heat-sensitive transfer ink layer having a surface resistivity of not more than 1013 xcexa9/xe2x96xa1.
In an embodiment of the present invention, the DBP oil absorption of the carbon black is from 100 to 150 ml/100 g.
In another embodiment of the present invention, the hot-melt ink contains an oil-soluble dye.
In still another embodiment of the present invntion, the carbon black is contained in the hot-melt ink in a range of 5 to 50 parts by weight with respect to 100 parts by weight of the hot-melt ink, and the oil-soluble dye is contained in a range of 1 to 30 parts by weight with respect to 100 parts by weight of the carbon black.
In a further embodiment of the present invention, a mat layer is provided between the substrate and the heat-sensitive transfer ink layer.